The invention relates to an ion source for a mass spectrometer and to a method of cleaning an ion source. Mass spectrometers normally operate at low pressure and the present invention is particularly concerned with an ion source which operates at atmospheric pressure. Such ion sources include electrospray ionisation (ESI) sources and atmospheric pressure chemical ionisation (APCI) sources.
Mass spectrometers have been used to analyse a wide range of materials, including organic substances, such as pharmaceutical compounds, environmental compounds and biomolecules. For mass analysis, it is necessary to produce ions of such sample compounds and biomolecules. Of particular use in the study of biological substances are mass spectrometers which have ion sources for creating ions of the sample compounds, where such ion sources operate at atmospheric pressure, or at least a pressure substantially higher than that of the mass spectrometer.
All atmospheric pressure ionisation (API) sources for mass spectrometers include an ion inlet orifice that forms a boundary between the API region and the low pressure region of the source or mass analyser.
This orifice is generally small (typically less than 0.5 mm in diameter) owing to the need to maintain a low pressure in the mass analyser region (typically less than 10xe2x88x924 mBar) and the finite pumping speed of the vacuum system used to maintain this low pressure.
The liquid chromatography (LC) inlet systems frequently used with these sources, e.g. APCI or electrospray probes, produce an aerosol in the atmospheric pressure region which, in addition to the gaseous sample ions, invariably contains involatile components that are infused either as chromatographic buffers or which appear in the analyte as sample extraction by-products.
As the sample ions pass from the high pressure region to the low pressure region through the orifice, these involatile components are deposited on the peripheral regions of the ion inlet orifice. Over prolonged periods of mass spectral analysis, this may eventually lead to a partial or complete blockage of the orifice and concomitant loss in sensitivity of the mass spectrometer with time.
Prior art API sources have utilised two alternative designs for the purpose of preventing the ion inlet orifice from being blocked due to the deposition of involatile substances, either a xe2x80x98sacrificialxe2x80x99 counterelectrode or an orthogonal source geometry.
FIG. 1 shows a typical counter electrode design. Here, the purpose of the counter electrode 2 is to present a surface 4 (a xe2x80x98sacrificialxe2x80x99 surface) for collecting excess involatile components which are within the aerosol produced by the probe 6. The gas flow (containing the ions and residual involatiles) is then redirected away from the direct line-of-sight of the orifice 20 to prevent the residual involatiles passing through the orifice 20 into the mass analyser 10 via the low pressure region 12 (which is maintained at a low pressure by pumps 8). However, over prolonged periods of use with strong chromatographic buffers (e.g. 50 mM sodium phosphate), these sources tend to lose sensitivity due to blockage of either the orifice 20 or the counter electrode 2 itself.
FIG. 2 shows a typical prior art orthogonal electrospray source design. The primary objective of this source geometry is to direct the spray away from the inlet orifice. However, at the higher flow rates used in LC mass spectroscopy (typically 1 ml/min), both the ions 22 and the charged liquid droplets 24 (containing involatile components) are deflected by the electric field towards the inlet orifice 20. This effect (which eventually leads to a blocked orifice) is shown schematically in FIG. 3a. 
A partial solution to this problem is effected by extending the position of the probe tip 6 towards the inlet orifice 20 as shown in FIG. 3b. In this case, the highly mobile ions 22 are still focused by the electric field into the orifice 20 whilst the high momentum liquid droplets 24 are deposited further downstream of the orifice.
Similarly, FIG. 3c shows a further improvement in source robustness obtained by reducing the electrospray potential, and hence the electric field between the probe and the orifice, which also has the effect of directing the large liquid droplets 24 away from the orifice 20.
However, these latter two improvements to the orthogonal geometry also lead to a significant reduction in sensitivity of the source.
A close inspection of the inlet orifice of an orthogonal geometry API source generally reveals that the majority of involatile components are deposited on the downstream cone surface and the downstream periphery of the orifice itself. This is shown schematically in FIG. 4. If the probe tip 6 is located to the upper left of the inlet orifice 20, then it is found that orifice blockage occurs due to crystallisation of involatile chromatographic buffers 26 on the lower edge of the orifice 20 and subsequent crystal growth upwards from this lower edge of the orifice 20.
The present invention aims to address the prior art problems of the deposition of involatiles and the resulting blockage of the orifice.
In one aspect, the present invention provides an ion source for a low pressure mass spectrometer comprising an atmospheric pressure sample ioniser operative at relatively higher pressure to provide a sample flow containing desired sample ions entrained with undesired gas and droplets, an orifice member defining an inlet orifice between the sample ioniser and the mass spectrometer, a conduit to transport a cleaning fluid, and a cleaning fluid reservoir suitable for connection to the conduit, the conduit having an opening adjacent the inlet orifice of the orifice member to dispense the cleaning fluid onto at least a portion of a surface of the orifice member during operation of the ion source.
Preferably the atmospheric pressure sample ioniser is operative to form a spray directed transversely of the axis of the inlet orifice, and the conduit opening is located to dispense the cleaning fluid onto a portion of the orifice member downstream of this orifice in the spray direction.
Advantageously, the conduit can have a plurality of openings adjacent to the inlet orifice of the orifice member for dispensing the cleaning fluid, the openings being positioned such that the entire periphery of the orifice is contacted by cleaning fluid. All of the surface adjacent to the orifice can then be cleaned, so as to prevent the build up of any materials on the surface that may result in blockage of the inlet orifice.
Preferably, the opening for dispensing the cleaning fluid can extend around the entire periphery of the orifice.
Preferably the orifice member is conical and the inlet orifice is formed at the apex of the cone.
Preferably the conduit is formed by a further conical member surrounding the cone of the orifice member and forming an annular opening surrounding the inlet orifice.
In another aspect, the present invention provides a method of cleaning the orifice member of an ion source for a low pressure mass spectrometer, the ion source comprising an atmospheric pressure sample ioniser operative at relatively higher pressure to provide a sample flow containing desired sample ions entrained with undesired gas and droplets, with an orifice member defining an inlet orifice between the sample ioniser and the mass spectrometer; the method comprising dispensing a cleaning fluid onto at least a portion of a surface of the orifice member adjacent the inlet orifice during the operation of the ion source.
Advantageously, the cleaning fluid can be continuously dispensed during operation of the ion source in order to prevent an accumulation of any substances that are deposited on the surface of the orifice member.
Preferably the cleaning fluid is dispensed on the surface of the orifice member on the higher pressure side thereof.
Advantageously the cleaning fluid can be dispensed so close to the inlet orifice that at least some of the dispensed cleaning fluid passes into the inlet orifice. This prevents the accumulation of any deposited involatile substances within the inlet orifice.
Advantageously, the cleaning fluid is dispensed around the entire periphery of the orifice.
Advantageously, the cleaning fluid is a solvent for the involatile components of the sample spray.